This present invention relates to cooking containers which can be used in microwave ovens, and to methods of manufacturing such containers. More particularly, the present invention relates to a container which provides improved microwave heating distributions when used in a microwave oven.
The invention will be particularly described with reference to the microwave cooking of foodstuffs, but it is to be understood that the invention in its broader aspect embraces the provision of containers (and methods of using them) for the microwave heating of bodies of any microwave-heatable material.
In applicant's copending U.S. patent application Ser. No. 878,171, filed June 25, 1986, and entitled "Microwave Container and Method of Making Same," now abandoned the disclosure of which is incorporated herein by this reference, there is described a container for containing a material to be heated in a microwave oven, this container comprising an open topped tray for carrying the material and a lid covering the tray to form a closed cavity, the container being characterized in that at least one surface of the container is formed with microwave generating means for generating a mode of a higher order than that of the fundamental modes of the container, the microwave generating means being so dimensioned and positioned with respect to the material when in the container that the mode so generated propagates into the material to thereby locally heat the material. As will be understood, in a container holding a food article being heated in a microwave oven, multiple reflections of radiation within the container or food article give rise to microwave field patterns which can be described as modes. It will also be understood that the term "generating" as used herein embraces both enhancement of modes already existing in the container and superimposition, on existing modes, of modes not otherwise existing in the container.
In a multi-compartment container, such as is used for heating several different foodstuffs simultaneously, the term "container" as used herein should be interpreted as meaning an individual compartment of that container. If, as is commonly the case, a single lid covers all compartments, then "lid" as used above means that portion of the lid which covers the compartment in question.
The container may be made primarily from metallic material, such as aluminum, or primarily from non-metallic material such as one of the various dielectric plastic or paperboard materials currently being used to fabricate microwave containers, or a combination of both.
In a conventional microwave oven, microwave energy, commonly at a frequency of 2.45 GHz, enters the oven cavity and sets up a standing wave pattern in the cavity, this pattern being at fundamental modes dictated by the size and shape of the walls of the oven cavity. In an ideal cavity, only fundamental modes exist, but in practice due to irregularities in the shape of the oven walls, higher order modes are also generated within the cavity and are superimposed on the fundamental modes. Generally speaking, these higher order modes are very weak, and in order to promote better distribution of energy within the container, a "mode stirrer" can be used to deliberately generate or enhance the higher order modes.
If a container, such as a food container, is placed in the microwave oven, and microwave energy is caused to propagate into the interior of that container, then a similar situation exists within the container as exists within the oven itself: a standing wave pattern is set up within the container, this pattern being primarily in the fundamental modes of the container (as distinct from the fundamental modes of the larger oven cavity), but also containing modes higher than those of the fundamental modes of the container, which higher modes are, for example, generated by irregularities in the interior shape of the container and its contents. As before, these higher order modes are generally of much lower power than the fundamental modes and contribute little to the heating of the material within the container.
Attention will now be directed to the manner in which the material within the container is heated by the microwave energy existing within the container. In doing this, it is convenient to study only horizontal planes within the container. It is well known that the standing wave pattern within the container consists of a combined electric and magnetic field. However, the heating effect is obtained only from the electric field and it is therefore of significance to examine the power distribution of the electric field as it exists under steady-state conditions within the container. In the fundamental modes--which, it should be recalled, are those predominantly existing within the container--the pattern of power distribution in the horizontal plane is confined to the edge of the container and this translates into a heating effect which is likewise concentrated around the edge of the container. The material in the central part of the container receives the least energy and therefore, during heating, its center tends to be cool. In conventional containers, this problem of uneven heating is ameliorated by instructing the user to leave the material unattended for a few minutes after the normal microwave cooking time in order for normal thermal conduction within the food to redistribute the heat evenly. Alternatively, the material may be stirred, if it is of a type which is susceptible to such treatment.
The shape of these "cold" areas varies according to the shape of the container. For example, for a rectangular container the shape of the cold area in the horizontal plane is roughly rectangular with rounded corners; for a container which is circular in horizontal cross section, the cold area will be likewise circular and positioned at the center of the container. For an irregularly shaped container, such as is commonly found in compartments of a multi-compartment container, the "cold" area will roughly correspond to the outside contour of the container shape and will be disposed centrally in the container.
In considering the heating effect of higher modes which may or may not exist within the container, it is necessary to notionally subdivide the container into cells, the number and arrangement of these cells depending upon the particular higher order mode under consideration. Each of these cells behaves, from the point of view of microwave power distribution, as if it were itself a container and therefore exhibits a power distribution which is high around the edges of the cell, but low in the center. Because of the physically small size of these cells, heat exchange between adjacent cells during cooking is improved and more even heating of the material results. However, in the normal container, i.e. unmodified by the structures described in the aforementioned copending application, these higher order modes are either not present at all or, if they are present, are not of sufficient strength to effectively heat the central regions of the food. Thus the primary heating effect is due to the fundamental modes of the container--i.e., a central cold area results.
Recognizing these problems, what the structures described in the aforementioned copending application seek to do, in essence, is to heat this cold area by introducing heating energy into the cold area. This can be achieved in two ways:
(1) by redistributing the microwave field pattern within the container by enhancing higher order modes which naturally exist anyway within the container due to the boundary conditions set by the physical geometry of the container, but not at an energy level sufficient to have a substantial heating effect or, where such naturally higher order modes do not exist at all (due to the geometry of the container), to generate such natural modes.
(2) to superimpose or "force" onto the normal field pattern--which, as has been said, is primarily in the fundamental modes--a further higher order field pattern whose characteristics owe nothing to the geometry of the container and whose energy is directed towards the geometric center of the container in the horizontal plane which is the area where the heating needs to be enhanced.
In both the above cases, the net result is the same: the container can be notionally considered as having been split into several smaller areas each of which has a heating pattern similar to that of the fundamental modes, as described above. However, because the areas are now physically smaller, normal thermal convection currents within the food have sufficient time, during the relatively short microwave cooking period, to evenly redistribute the heat and thus avoid cold areas. In practice, under certain conditions higher order mode heating may take place due to both of the above mechanisms simultaneously.
The process for generating the microwave field, as described in the aformentioned copending application, may take one of two forms:
(1) Where said at least one surface of the container takes the form of a sheet of microwave transparent material, a plate of electrically conductive material which is attached to or forms part of the sheet. Such a plate could be made for example of aluminum foil which is adhered to the sheet, or could be formed as a layer of metallization applied to the sheet.
(2) Where said at least one surface of the container takes the form of a sheet of electrically conductive material, such as aluminum foil, an aperture in the sheet through which microwave energy incident on the sheet can pass. Preferably, the aperture is covered by microwave transparent material. In some instances, however, the aperture may simply be a void (i.e. open), for example to permit venting of steam from within the container.
It will be appreciated that the two alternatives listed above--i.e., the plate and the aperture--are analogues of one another. For ease of understanding, in the first alternative, the plate can be considered as a two-dimensional antenna, the characteristics of which follow from well-known antenna theory. Thus, the plate can be considered as receiving microwave energy from the oven cavity, whereupon a microwave field pattern is set up in the plate, the characteristics of which pattern are dictated by the size and shape of the plate. The plate then retransmits this energy into the interior of the container as a microwave field pattern. Because the dimensions of the plate are necessarily smaller than those of the container surface with which it is associated, the order of the mode so transmitted into the interior will be higher than the container fundamental modes.
In the second alternative, the aperture can be considered as a slot antenna, the characteristics of which again follow from theory. The slot antenna so formed effectively acts as a window for microwave energy from the oven cavity. The edges of the window define a particular set of boundary conditions which dictate the microwave field pattern which is formed at the aperture and transmitted into the interior of the container. Once again, because the dimensions of the aperture are smaller than those of the container surface with which it is associated, the shape and (particularly) the dimensions of the aperture are such as to generate a mode which is of a higher order than the container fundamental modes.
Several separate higher order mode generating means--be they plates or apertures--may be provided on each container to improve the heat distribution. The higher order mode generating means may all be provided on one surface of the container, or they may be distributed about the container on different surfaces. The exact configuration will depend upon the shape and normal (i.e., unmodified by the plates and/or apertures) heating characteristics, the object always being to get microwave energy into the cold areas, thus electrically subdividing the container down into physically smaller units which can more readily exchange heat by thermal conduction. The considerations which are to be given to the positioning of the higher order mode generating means will depend upon which of the two mechanisms of operation it is desired to use: if it is desired to enhance or generate a particular higher order mode which is natural to the container, then the above-mentioned cell pattern appropriate to that mode should be used to position the plates or apertures forming the higher order mode generating means. In order to enhance or generate a natural mode, a plate/aperture of approximately the same size as the cell will need to be placed over at least some of the cells--the larger the number of cells which have a plate or aperture associated with them, the better the particular mode chosen will be enhanced. In practice, a sufficient space must be left between individual plates/apertures in order to prevent field interaction between them--it is important that each plate/aperture is sufficiently far from its neighbor to be able to act independently. If the spacing is too close, the incident microwave field will simply see the plates/apertures as being continuous and, in these circumstances, the fundamental mode will predominate, which will give, once again, poor heat distribution. A typical minimum spacing between plates would be in the range of 6 to 12 mm, depending upon the particular container geometry and size. A typical minimum spacing between apertures (i.e. where the apertures are separated by regions of foil or other metallized layer) is in the range of 6 to 12 mm., both to protect the electrical integrity of the structure from mechanical damage such as scratches and to avoid ohmic overheating which is likely to result from high induced currents in narrower metal strips; a typical minimum width of metal border regions defining the outer peripheries of apertures would be in the same range, for the same reasons.
If, on the other hand, it is desired to use the mechanism of "forcing" an unnatural higher order mode into the container, then the plate/aperture forming the higher mode generating means needs to be placed over the cold area or areas within the container. In such circumstances, the plate/aperture, in effect, acts as a local heating means and does not (usually) significantly affect the natural modes of the container. Thus the "forced" mechanism utilizes the heating effect of the container fundamental superimposed onto its own heating effect. At certain critical sizes and positioning of the plates, both mechanisms--forced and natural--may come into play.
For convenience of explanation, the present discussion considers matters only in the horizontal plane and for the same reason, the only surfaces which are formed with the higher order generating means in the embodiments which follow are horizontal surfaces--i.e., the bottom of the container or the lid of the container. However, there is no reason why the teachings of the aforementioned copending application (and of the present invention) should not be applied to other than horizontal surfaces since the ambient microwave field in which the container is situated is substantially homogeneous.
Because the characteristics of the plate/aperture alternatives are analogous (indeed a particular aperture will transmit an identical mode to that transmitted by a plate of identical size and shape), it is possible to use them interchangeably--in other words, whether a plate or aperture of particular dimensions is used, can be dictated by considerations other than that of generating a particular microwave field pattern.
Clearly, the heating effect of the higher order mode generating means will be greatest in the food immediately adjacent to it and will decrease in the vertical direction. Thus, it may be an advantage to provide higher mode generating means both in the lid and in the bottom of the container. Since the cold areas will be in the same position in the horizontal plane whether the lid or the bottom of the container is being considered, it is clearly convenient to make the higher mode generating means in the lid in registry with those in the bottom of the container. By this means, better heat distribution in the vertical direction can be achieved. It matters not which particular type of higher mode generating means is used as between the lid and the bottom--in one embodiment, for example, a plate or plates are formed on the lid, while in-registry aperture or apertures are formed in the container bottom. In another embodiment, apertures are provided in both lid and bottom surfaces.
The aforementioned copending application also contemplates a method of manufacturing a container as described above for containing a material to be heated in a microwave oven, comprising forming, on at least one surface of the container, microwave generating means for generating a mode of a higher order than that of the fundamental modes of the container, such generating means being so dimensioned and positioned with respect to the material when in the container that the mode so generated propagates into the material to thereby locally heat the material. Each higher order mode generating means may be so configured and positioned on its surface as to generate or amplify higher order modes which are natural to the container and dictated by its boundary conditions, and/or to generate a mode which is of higher order than that of the fundamental of the container but is not otherwise dictated by the boundary conditions of the container and would not normally exist therein.